SYNOPSIS

DESCRIPTION

Linux 2.2 optionally implements the Internet Protocol, version 6. This
man page contains a description of the IPv6 basic API as implemented by
the Linux kernel and glibc 2.1. The interface is based on the BSD
sockets interface; see socket(7).
The IPv6 API aims to be mostly compatible with the ip(7) v4 API. Only
differences are described in this man page.
To bind an AF_INET6 socket to any process the local address should be
copied from the in6addr_any variable which has in6_addr type. In
static initializations IN6ADDR_ANY_INIT may also be used, which expands
to a constant expression. Both of them are in network order.
The IPv6 loopback address (::1) is available in the global
in6addr_loopback variable. For initializations IN6ADDR_LOOPBACK_INIT
should be used.
IPv4 connections can be handled with the v6 API by using the v4-mapped-
on-v6 address type; thus a program only needs only to support this API
type to support both protocols. This is handled transparently by the
address handling functions in libc.
IPv4 and IPv6 share the local port space. When you get an IPv4
connection or packet to a IPv6 socket its source address will be mapped
to v6 and it will be mapped to v6.
AddressFormat
struct sockaddr_in6 {
uint16_t sin6_family; /* AF_INET6 */
uint16_t sin6_port; /* port number */
uint32_t sin6_flowinfo; /* IPv6 flow information */
struct in6_addr sin6_addr; /* IPv6 address */
uint32_t sin6_scope_id; /* Scope ID (new in 2.4) */
};
struct in6_addr {
unsigned char s6_addr[16]; /* IPv6 address */
};
sin6_family is always set to AF_INET6; sin6_port is the protocol port
(see sin_port in ip(7)); sin6_flowinfo is the IPv6 flow identifier;
sin6_addr is the 128-bit IPv6 address. sin6_scope_id is an ID of
depending of on the scope of the address. It is new in Linux 2.4.
Linux only supports it for link scope addresses, in that case
sin6_scope_id contains the interface index (see netdevice(7))
IPv6 supports several address types: unicast to address a single host,
multicast to address a group of hosts, anycast to address the nearest
member of a group of hosts (not implemented in Linux), IPv4-on-IPv6 to
address a IPv4 host, and other reserved address types.
The address notation for IPv6 is a group of 16 2 digit hexadecimal
numbers, separated with a ’:’. ’::’ stands for a string of 0 bits.
Special addresses are ::1 for loopback and ::FFFF:<IPv4 address> for
IPv4-mapped-on-IPv6.
The port space of IPv6 is shared with IPv4.
SocketOptions
IPv6 supports some protocol-specific socket options that can be set
with setsockopt(2) and read with getsockopt(2). The socket option
level for IPv6 is IPPROTO_IPV6. A boolean integer flag is zero when it
is false, otherwise true.
IPV6_ADDRFORM
Turn an AF_INET6 socket into a socket of a different address
family. Only AF_INET is currently supported for that. It is
only allowed for IPv6 sockets that are connected and bound to a
v4-mapped-on-v6 address. The argument is a pointer to an
integer containing AF_INET. This is useful to pass v4-mapped
sockets as file descriptors to programs that don’t know how to
deal with the IPv6 API.
IPV6_ADD_MEMBERSHIP,IPV6_DROP_MEMBERSHIP
Control membership in multicast groups. Argument is a pointer
to a structipv6_mreq structure.
IPV6_MTU
Set the MTU to be used for the socket. The MTU is limited by
the device MTU or the path mtu when path mtu discovery is
enabled. Argument is a pointer to integer.
IPV6_MTU_DISCOVER
Control path mtu discovery on the socket. See IP_MTU_DISCOVER
in ip(7) for details.
IPV6_MULTICAST_HOPS
Set the multicast hop limit for the socket. Argument is a
pointer to an integer. -1 in the value means use the route
default, otherwise it should be between 0 and 255.
IPV6_MULTICAST_IF
Set the device for outgoing multicast packets on the socket.
This is only allowed for SOCK_DGRAM and SOCK_RAW socket. The
argument is a pointer to an interface index (see netdevice(7))
in an integer.
IPV6_MULTICAST_LOOP
Control whether the socket sees multicast packets that it has
send itself. Argument is a pointer to boolean.
IPV6_PKTINFO
Set delivery of the IPV6_PKTINFO control message on incoming
datagrams. Only allowed for SOCK_DGRAM or SOCK_RAW sockets.
Argument is a pointer to a boolean value in an integer.
IPV6_RTHDR,IPV6_AUTHHDR,IPV6_DSTOPS,IPV6_HOPOPTS,IPV6_FLOWINFO,IPV6_HOPLIMIT
Set delivery of control messages for incoming datagrams
containing extension headers from the received packet.
IPV6_RTHDR delivers the routing header, IPV6_AUTHHDR delivers
the authentication header, IPV6_DSTOPTS delivers the destination
options, IPV6_HOPOPTS delivers the hop options, IPV6_FLOWINFO
delivers an integer containing the flow ID, IPV6_HOPLIMIT
delivers an integer containing the hop count of the packet. The
control messages have the same type as the socket option. All
these header options can also be set for outgoing packets by
putting the appropriate control message into the control buffer
of sendmsg(2). Only allowed for SOCK_DGRAM or SOCK_RAW sockets.
Argument is a pointer to a boolean value.
IPV6_RECVERR
Control receiving of asynchronous error options. See IP_RECVERR
in ip(7) for details. Argument is a pointer to boolean.
IPV6_ROUTER_ALERT
Pass forwarded packets containing a router alert hop-by-hop
option to this socket. Only allowed for SOCK_RAW sockets. The
tapped packets are not forwarded by the kernel, it is the user’s
responsibility to send them out again. Argument is a pointer to
an integer. A positive integer indicates a router alert option
value to intercept. Packets carrying a router alert option with
a value field containing this integer will be delivered to the
socket. A negative integer disables delivery of packets with
router alert options to this socket.
IPV6_UNICAST_HOPS
Set the unicast hop limit for the socket. Argument is a pointer
to an integer. -1 in the value means use the route default,
otherwise it should be between 0 and 255.

VERSIONS

The older libinet6 libc5 based IPv6 API implementation for Linux is not
described here and may vary in details.
Linux 2.4 will break binary compatibility for the sockaddr_in6 for
64-bit hosts by changing the alignment of in6_addr and adding an
additional sin6_scope_id field. The kernel interfaces stay compatible,
but a program including sockaddr_in6 or in6_addr into other structures
may not be. This is not a problem for 32-bit hosts like i386.
The sin6_flowinfo field is new in Linux 2.4. It is transparently
passed/read by the kernel when the passed address length contains it.
Some programs that pass a longer address buffer and then check the
outgoing address length may break.

NOTES

The sockaddr_in6 structure is bigger than the generic sockaddr.
Programs that assume that all address types can be stored safely in a
structsockaddr need to be changed to use structsockaddr_storage for
that instead.

BUGS

The IPv6 extended API as in RFC 2292 is currently only partly
implemented; although the 2.2 kernel has near complete support for
receiving options, the macros for generating IPv6 options are missing
in glibc 2.1.
IPSec support for EH and AH headers is missing.
Flow label management is not complete and not documented here.
This man page is not complete.